What is MSG?
MSG, short name for Message, is used as the file extension for Outlook Message Item files.
.MSG files are binary files that can be opened by Microsoft Outlook software.
See the sample .MSG file included below.

✍: FYIcenter.com

A

File Extension: .MSG

MIME Type: application/octet-stream

File Content: Outlook Message Item

.MSG files are Outlook Message files saved as binary files.
Here are the first 64 bytes of a sample .MSG file dumped in HEX values: